Conventionally, signal lights have been proposed that use as the light source a number of LEDs arrayed in a plane on a board for the purpose of energy saving. LEDs, however, have a low electricity-to-light conversion efficiency and change most of the electricity to heat despite of their high illumination efficiency and energy saving effectiveness. When used collectively in a large number at a time, therefore, LEDs generate a substantially large calorific power. In addition, LEDs have a negative temperature coefficient of light emission, so that their illumination decreases as the ambient temperature rises; for example, the light intensity is halved at an ambient temperature of 80.degree. C. as compared with that at 25.degree. C. To keep a high illumination efficiency of LEDs, therefore, their own temperature must be kept low. The abovementioned conventional signal light which uses many LEDs has such a fault that the LEDs arrayed as many as meet a prescribed illumination standard as the light source generate a substantially large calorific power when they are energized at the same time. As a result, not only the illumination efficiency of the LEDs will lower but also their service life will shorten, thus resulting in a failure as a signal light unit.
This invention solves the problem of such fault with a purpose of providing a signal light unit that has a heat dissipating function that can dissipate effectively the heat generated inside the LEDs to open air through a heat sink located on the back face of the board on the right face of which many LEDs are arrayed in a plane, thus preventing lowering of the light emission efficiency of the LEDs by heat and reverse effects of heat on the signal light.